1. Field of the Invention
The present invention relates to a screen printing apparatus using a heat-sensitive screen master.
2. Description of the Prior Art
In the heat-sensitive screen printing, it is known to use a printing plate 5 which includes a screen master 3 (FIG. 6 of the accompanying drawings) composed of a thermoplastic synthetic resin film 1 and a porous screen 2 stuck together, and a centrally opening frame 4 (FIG. 5) attached to the screen master 3 on its screen side so as to stretch the screen master 3 with an appropriate degree of tension. For plate-making or stencil-making, as shown in FIG. 7, an original 6 with its picture or image side down is placed on the thermoplastic synthetic resin film 1 of the printing plate 5, and then the printing plate 5 is set on a plate or stencil making apparatus 7 in such a manner that the frame 4 of the printing plate 5 contacts a transparent support 8 of the stencil making apparatus 7. Subsequently, the screen master 3 and the original 6 are pressed by a pressure pad 9 against the transparent support 8 so as to tightly contact each other, whereupon light from a light source 10 is irradiated to the screen master 3. As a result, the exposed areas of the original 6 are formed to appear as an image in pores on the thermoplastic synthetic resin film 1 of the screen master 3. The thus processed printing plate 5 is used as a stencil. For printing, as shown in FIG. 8, the processed printing plate 5 with its screen side up is placed on a print paper 11 supported on a table (not shown), and an amount of ink 12 is spread by a squeegee 13 over the screen 2 within the frame 4.
However, a primary problem with such prior art is in that it is difficult or requires a well-experienced workperson to attach the frame 4 to the screen master 3 so as to impart thereto an appropriate degree of tension. If the screen master 3 could not be tightly stretched by the frame 4, such screen master 3 would easily become relaxed when it is depressed toward the transparent support 8 by the pressure pad 9. This is true because the screen master 3 must be depressed from the position of FIG. 7 in which it is spaced from the transparent support 8 by a distance equal to the thickness of the frame 4. When the ink 12 is spread over this relaxed screen master 3 by the squeegee 13, a shear in printing would occur and hence an objectionably elongated or warped image would appear on the print paper 11.